This invention relates in general to photonics, and in particular to the modulation of photonic structures for achieving non-reciprocal optical effects for various applications, such as optical isolation.
Achieving on-chip optical signal isolation is a fundamental difficulty in integrated photonics, see Soljacic, M. & Joannopoulos, J. D. “Enhancement of nonlinear effects using photonic crystals,” Nature Material 3, 211-219 (2004). The need to overcome this difficulty, moreover, is becoming increasingly urgent, especially with the emergence of silicon nano-photonics, see Pavesi, L. & Lockwood, Silicon Photonics (Springer, Berlin, 2004), Almeida, V. R. Barrios, C. A. Panepucci, R. R. & Lipson, M. All-optical control of light on a silicon chip. See, Nature, 431, 1081-1084 (2004), Miller, D. A. B. “Optical interconnects to silicon,” IEEE J. Sel. Top. Quant. Electron. 6, 1312-1317 (2000), which promise to create on-chip optical systems at an unprecedented scale of integration. In spite of many efforts, there have been no techniques that provide complete on-chip signal isolation using materials or processes that are fundamentally compatible with silicon CMOS process. Here we introduce an isolation mechanism based on indirect interband photonic transition. Photonic transition, as induced by refractive index modulation, see Winn, J. N. Fan, S. Joannopoulos, J. D. & Ippen, E. P. “Interband transitions in photonic crystals,” Phys. Rev. B 59, 1551-1554 (1998), has been recently observed experimentally in silicon nanophotonic structures, see Dong, P. Preble, S. F. Robinson, J. T. Manipatruni, S. & Lipson, M. “Inducing photonic transitions between discrete modes in a silicon optical microcavity,” Phys. Rev. Lett. 100, 033904 (2008). Here we show that a linear, broad-band, and non-reciprocal isolation can be accomplished by spatial-temporal modulations that simultaneously impart frequency and wavevector shifts during the photonic transition process. We further show that non-reciprocal effect can be accomplished in dynamically-modulated micron-scale ring-resonator structures.
To create complete optical signal isolation requires time-reversal symmetry breaking. In bulk optics, this is achieved using materials exhibiting magneto-optical effects. Despite many efforts however see Espinola, R. L. Izuhara, T. Tsai, M.-C. Osgood, R. M. Jr. & Dötsch, H. “Magneto-optical nonreciprocal phase shift in garnet/silicon-on-insulator waveguides,” Opt. Lett. 29, 941-943 (2004), Levy, M. “A nanomagnetic route to bias magnet-free, on-chip Faraday rotators,” J. Opt. Soc. Am. B 22, 254-260 (2005), Zaman, T. R. Guo, X. & Ram, R. J. “Faraday rotation in an InP waveguide,” Appl. Phys. Lett. 90, 023514 (2007), Dotsch, H. et al. “Applications of magneto-optical waveguides in integrated optics: review,” J. Opt. Soc. Am. B 22, 240-253 (2005), on-chip integration of magneto-optical materials, especially in silicon in a CMOS compatible fashion, remains a great difficulty. Alternatively, optical isolation has also been observed using nonlinear optical processes, see Soljaic, M. Luo, C. Joannopoulos, J. D. & Fan, S. “Nonlinear photonic microdevices for optical integrations,” Opt. Lett. 28, 637-639 (2003), Gallo, K. Assanto, G. Parameswaran, K. R. and Fejer, M. M. “All-optical diode in a periodically poled lithium niobate waveguide,” Appl. Phys. Lett. 79, 314-316 (2001), or in electro-absorption modulators, see Ibrahim, S. K. Bhandare, S. Sandel, D. Zhang, H. & Noe, R. “Non-magnetic 30 dB integrated optical isolator in III/V material,” Electron. Lett. 40, 1293-1294 (2004). In either case, however, optical isolation occurs only at specific power ranges, see Soljaic, M. Luo, C. Joannopoulos, J. D. & Fan, S. “Nonlinear photonic microdevices for optical integrations,” Opt. Lett. 28, 637-639 (2003), Gallo, K. Assanto, G. Parameswaran, K. R. and Fejer, M. M. “All-optical diode in a periodically poled lithium niobate waveguide,” Appl. Phys. Lett. 79, 314-316 (2001), or with associated modulation side bands, see Ibrahim, S. K. Bhandare, S. Sandel, D. Zhang, H. & Noe, R. “Non-magnetic 30 dB integrated optical isolator in III/V material,” Electron. Lett. 40, 1293-1294 (2004). In addition, there have been works aiming to achieve partial optical isolation in reciprocal structures that have no inversion symmetry (for example, chiral structures). In these systems, the apparent isolation occurs by restricting the allowed photon states in the backward direction, and would not work for arbitrary backward incoming states. None of the non-magnetic schemes, up to now, can provide complete optical isolation.